In the field of sewing machines, a device for automatically varying the pressure applied on a workpiece cloth by a presser foot is known that varies the pressure with the variation in machine speed typically represented by SPM (Stitches Per Minute). Such device controls the pressure imparted by the presser foot to increase with machine speed in order to suppress presser foot jumping which intensifies with machine speed.
In operation, sewing machine pierces the workpiece cloth with a threaded sewing needle and forms a needle thread loop at the needle eye as the sewing needle is lifted out of the workpiece cloth. Needle thread loop is a small loop formed by the needle thread running between the needle eye and the workpiece cloth. The needle thread loop is seized by the hook beak to allow the bobbin thread to be passed through it to create a seam on the workpiece cloth with the interlaced needle thread and bobbin thread.
In sewing machines that are provided with a needle bar swing mechanism that laterally swings the needle bar, hook beak meets the sewing needle in different timings when needle bar is placed in the left baseline position (i.e. left needle drop position) and the right baseline position (i.e. right needle drop position). Meeting or encounter of the hook beak and the sewing needle in this context indicates the timing in which the tip of the beak overlaps with the sewing needle when viewed from the front side of the sewing machine. The beak seizes the needle thread loop at this timing and thus, this timing can also be deemed as the meeting point of the beak and the needle thread loop. Normally, the beak meets the sewing needle at an earlier timing when the needle bar is placed in the right baseline position as compared to the left baseline position. The size of the needle thread loop relies on the amount of elevation the sewing needle pierced through the workpiece cloth travels until it meets the beak. Thus, the size of the needle thread loop varies depending upon the position in which the needle bar is swung.
Such being the case, the needle thread loop may become oversized in certain needle swing positions and may sag or topple by gravity to lose its shape. This disables the loop seizure on the part of the beak to cause sew errors such as skipped stitches
As mentioned earlier, the size of the needle thread loop relies on the amount of elevation the sewing needle pierced through the workpiece cloth travels until it meets the beak. When raising the sewing needle, the workpiece cloth must be pressed down by the presser foot. Failure to do so will result in the elevation of the workpiece cloth with the sewing needle which will not allow the needle thread loop to form. The inventors of the present disclosure conceived of adjusting the size of the needle thread loop by slightly reducing the pressure applied by the presser foot, that is, allowing the elevation of the workpiece cloth W in controlled amounts with the rising of the sewing needle. The inventors have realized such loop size adjustment feature through control of the presser foot lifting/lowering mechanism to provide a sewing machine that is preventive of skipped stitches.